1. Field of the Invention
This invention is in the field of forming bicast metal structures by means of positioning a preform in a mold and casting the metal to be joined thereto about selected portions of the preform. The improvement of the present invention involves the use of an intermediate layer between the preform and the cast metal, said layer acting as an aid in bonding the two portions during casting and, by virtue of its special characteristics, further enhancing the strength of the bond after subsequent heat treatment.
2. Description of the Prior Art
Bimetallic casting processes, per se, have been described in prior art literature and patents. By and large these processes are directed to the production of a metallurgical bond between the preform and the metal which is cast about it. Reference is invited to Schwartz et al. U.S. Pat. Nos. 3,279,006 and 3,342,564 as examples of such disclosures. These patents describe the production of composite metallic objects by melting a metallic material having a specific property desired in the poured portion thereof under vacuum, heating a refractory mold having a cavity therein adapted to receive melted metallic material and having a solid metallic object or element positioned therein with at least a portion of the surface thereof exposed within the cavity, under vacuum and pouring the molten metallic material while maintaining an inert atmosphere. The bonding which exists between the solidified molten metal and the preform results from the interalloying of the preform with the poured metal to produce a metallurgically bonded zone.
While metallurgical bonding is an effective means for joining the two portions together into a bimetallic article, such bonds are difficult to achieve on a reliable and reproducible basis. In practice very high vacuum levels or other inert atmospheres must be used to prevent formation of contaminants at the interfacial region which can reduce the level of bonding. The temperatures of the pre-existing portion and the molten metal must be such that neither too rapid cooling of the cast-on metal occurs, which could reduce bond strength by preventing sufficient interalloying, or excessively slow cooling occurs, which could lead to gross melting of the initial solid portion. The physical contact occurring between the two materials being joined is characterized by extreme proximity, being aided not only by the capability of molten metal to fill even microscopic recesses in the initial solid portion but by the relatively greater contraction occurring in the cast-on material by virtue of its solidifying and subsequently cooling from a greater temperature than the initial solid material. The resulting physical contact precludes, except in the case of gross separation, the nondestructive inspection of the article for bond quality.
It has been found that mold preheating and casting conditions utilized to produce a bicast article can be such that regions showing metallurgical bonding and areas free of a metallurgical bond can both exist in the attachment region. If the design of the article for satisfactory operation in service relies exclusively on a metallurgical bond, undetectable areas of inadequate bonding can lead to premature failure.
Reference is made to the prior technical information published in the literature, (Article by U. Okapuu and G. S. Calvert entitled "An Experimental Cooled Radial Turbine" appearing in Agard Conference Proceedings No. 73 on High Temperature Turbines, Agard-CP-73-71, Paper No. 10, January, 1971) in which a gas turbine rotor was produced by bicasting a nickel-base superalloy hub around root areas of previously cast nickel-base superalloy blades. The design was based on achieving a metallurgical bond although a few small recesses were provided to yield some mechanical support. The root areas were tapered in a manner which, in the absence of the recesses and any metallurgical bond, would permit unrestricted removal of the blades from the hub portion. In practice use of vacuum preheating and pouring conditions based on prior controlled tests using castings which modeled the attachment resulted in the metallurgical bonding being limited to a single region of the root area. The resultant performance of the bimetallic part was not wholly adequate by virtue of failures from debonding at areas not fully bonded metallurgically.
The degree of bonding necessary to achieve the necessary degree of integrity in a metallurgical sense is such that a zone of alloying due to local intermelting or diffusion occurs without a discrete interface containing weakening constituents. Even the presence of a thin film of a weakening constituent having a thickness of only 0.00001 inch or even less can be sufficient to prevent bonding. In some metals and alloys, severe reductions in mechanical strength and ductility are known to occur from films only a few atomic layers thick between grains. These can arise from compositional impurities, improper metal working or casting procedures, heat treatments or various combinations of these.